What type of bird is often referred to as a 'passage' bird?

A 'passage' bird typically refers to an adult bird that is in the process of migrating between its breeding and wintering grounds. The term is commonly used in the context of birds of prey, particularly falcons. During migration, these birds are often caught during their journey from one region to another. Understanding that a passage bird is an adult emphasizes its capability to survive the migration process and establish the behaviors associated with breeding and foraging. This aspect is fundamental in falconry, as these birds are often sought after for hunting due to their experience and developed hunting skills.

In contrast, the other options describe different stages or circumstances of birds that do not align with the definition of a 'passage' bird. For example, a young bird that hasn’t yet fledged refers to a juvenile that is still in the nest and not capable of migration, while a bird that has been captured and trained relates more to exploitation in falconry rather than its migratory behavior. Lastly, a bird born in captivity is not a passage bird since it has never engaged in the migration process typical of wild birds.